116 
The Rot in Sheep. 
Fifj. 11 represents on a large scale the development of tlie 
ccrcarid' as it goes on within the sac from the first bud or sporule to 
the perfect embryo. A reference to the figures will show — 1, a 
sporule ; 2, sporule elongating ; 3, sporule becoming caudated ; 
4, early form of cercaria ; and 5, perfect embryo. In the last- 
named figure, a indicates the oral aperture ; c, d, the urinary 
organ ; e, tlie tail ; and two pigment-spots. 
When first set free from the sac the cercaria is rather tardy in 
its action, but after a time it swims freely about, assisted in its 
various movements by the length of its tail. Fig. 12, which we 
Fig. 12. 
A fully developed Cercaria ejjhemera. After Huxley. 
here insert, shows a fully developed Cercaria ephemera, the body 
of which, it will be noticed, bears a strong resemblance to a 
fluke. In this figure, a represents the mouth ; h, the alimentary 
canal ; c, d, the urinary organ ; e, the tail ; and f, pigrhent- 
spots. It is worthy of note that in the most perfected cercaria: 
no sexual organs can be detected, although in other respects 
their resemblance to distomata is so complete. It is evident 
from this that they have to undergo a higher form of develop- 
ment, which they can only attain by becoming entozoic to 
other creatures. Some varieties of them have been observed to 
